In recent years, there has been a surge of interest in erbium-doped solid state lasers emitting near 3 microns for medical procedures because radiation at this wavelength is strongly absorbed by water contained in biological tissue. In the absence of pigment, water acting as a chromophore largely determines the light absorption properties of the respective tissue. For example, the absorption coefficient of water around 3 .mu.m is more than four orders of magnitude higher than the absorption near the common 1 .mu.m wavelength, resulting in shallow penetration depths and highly localized interactions at these mid-infrared wavelengths.
One solid state laser in particular, the erbium-doped YAG (Er:YAG) laser, emits radiation at a wavelength of 2.94 .mu.m, virtually at the peak of the water absorption curve. With an absorption coefficient of over 13000 cm.sup.-1, this laser can produce a very small region of impact with potentially less than one or two microns penetration depth. Since the ablation threshold and extension of thermal injury are inversely proportional to the absorption coefficient, the Er:YAG's emission can be particularly effective in certain surgical applications including delicate endoscopic procedures, micro-ocular surgery and corneal sculpting, all of which require a high degree of precision and control combined with minimal damage to tissue adjacent to the surgical site.
One particular application generating growing interest in the past few years involves the procedure of photorefractive keratectomy (PRK) for reshaping the cornea of the eye. PRK techniques based on volumetric removal of tissue using ultraviolet (UV) radiation, typically from a 193 nm ArF excimer laser, have become widely utilized as an effective means for correcting visual deficiencies. At this short wavelength, the high photon energy causes direct breaking of intramolecular bonds, a process known as photochemical decomposition. Tissue ablation based on this photochemical mechanism has the advantage of producing minimal collateral thermal damage in cells adjacent to the surgical site. Also, the depth of decomposition is very small, typically less than 1 micron, resulting in accurate tissue removal with minimal risk of damage to underlying structures from the UV radiation.
While established by the Food and Drug Administration (FDA) in the United States as a safe and effective method of corneal ablation, excimer based methods also suffer from a number of deficiencies, including high initial and maintenance costs, large and complex optical beam delivery systems, safety hazards due to the fluorine and ozone gas formation and persistent reliability problems. Furthermore, the potential phototoxicity of high power UV radiation is still an undetermined risk in excimer-laser-based PRK. In particular, there is concern that the UV radiation poses certain mutagenic and cataractogenic risks due to secondary fluorescence effects.
Ablation at mid-infrared wavelengths using, especially radiation around 3 .mu.m, has been suggested as an alternative to the excimer laser for performing corneal refractive surgery. With water being the main constituent of the cornea, radiation corresponding to the absorption peak of water has the potential to ablate tissue selectively with minimal collateral thermal damage, similar to what is produced with the excimer laser. The premise underlying interest in such an alternative system is that infrared radiation can be produced with an all-solid-state technology which would provide easier handling, be cheaper, more compact and have better reliability features, while eliminating the potential of any safety concerns due to toxic gases or mutagenic side effects associated with deep UV wavelengths. The fact that there was a solid state laser fortuitously emitting radiation with the desired wavelength, namely, the Er:YAG laser, contributed to the interest in exploiting a controlled thermal mechanism for this application.
Contrary to the photoablation mechanism associated with the excimer laser, i.e., photochemical decomposition, ablation in the infrared wavelength range is generally attributed to photothermal vaporization. This process inherently has a larger effect than photodecomposition, allowing for removal of up to several microns of tissue per pulse at a time, thereupon resulting in faster surgical operations, but also with a larger thermal damage zone. A system for performing PRK based on a photovaporization process has been suggested, for example, by T. Seiler and J. Wollensak, in "Fundamental Mode Photoablation of the Cornea for Myopic Correction", Lasers and Light in Ophthalmology, 5, 4, 199-203 (1993). Another system has been described by Cozean et al. in PCT Application No. 93/14817, which relies on a sculpting filter to control the amount of tissue removal using a pulsed 3 .mu.m Er:YAG laser. However, while ophthalmic surgical techniques based on such free-running or long-pulse erbium lasers have shown some promise, they also suffer from a number of drawbacks principally relating to the fact that the IR radiation causes collateral thermal damage to tissue adjacent to the ablated region. In fact, the size of the damage zone with such systems may be up to 50 microns, leading to potentially undesirable short and long term healing side effects such as haze, regression, and loss of visual acuity.
Recently, it has been recognized that mid-infrared lasers emitting shorter pulses, for example, utilizing Q-switched lasers, cause less thermal damage. For a review of mid-infrared laser systems, see, for example, Q. Ren et al in Opt. Eng., 34, pp. 642-660, 1995. However, even with pulses this short, on the order of hundreds of nanoseconds, compared to hundreds of microseconds for previous studies, the collateral damage zone still extends up to 21 .mu.m. See, for example, J. Lian & K. Wang in SPIE., 2393, pp. 160-166, 1995. Since such extended thermal damage zones are still accompanied by haze, regression, and other deleterious healing side-effects, this puts infrared lasers at a disadvantage when compared with excimer lasers for corneal ablation.
More recently, a direct tissue interaction effect known as photospallation, or photomechanical ablation, has been observed at infrared wavelengths, whereby, with pulses shorter than 50 nanoseconds and preferably shorter than 30 ns, radiation interacts exclusively with the irradiated tissue producing negligible effect upon the adjacent, unirradiated tissue. Photospallation is a photomechanical ablation mechanism (distinctly different from both photothermal vaporization and photochemical decomposition), resulting from the rapid absorption of incident radiation and subsequent expansion by the corneal tissue. This expansion is followed by a bi-polar shock wave which causes removal of tissue. This process was originally described in Jacques, S. L. "Laser-Tissue Interactions: Photochemical, Photothermal, and Photomechanical," Lasers in General Surgery, 72(3),531-558 (1992) and was recently observed in animal experiments conducted with short pulse mid-infrared scanning laser delivery systems constructed according to principles similar to those described in our U.S. patent application Ser. No. 08/549,385, which is incorporated by reference herein.
Generally, U.S. patent application Ser. No. 08/549,385 discloses a method and apparatus for performing corneal surgery utilizing a short-pulse (less than 50 ns) solid state laser emitting mid-infrared radiation, preferably at or around 2.94 .mu.m, scanned over a region of the cornea to allow uniform irradiation of the treatment region using a relatively low-energy laser (less than 30 mJ). By taking advantage of the lower energy density threshold (defined as the lowest fluence at which ablation occurs) associated with the photospallation process, tissue would thus be removed more accurately and safely than with prior art methods and apparatus. As one example of such prior art, ablation thresholds ranging from 200 to 600 mJ/cm.sup.2 were recited by J. T. Lin in U.S. Pat. No. 5,520,679 as necessary for corneal sculpting application at mid-infrared wavelengths, including 2.94 .mu.m. It was pointed out in the Lin patent, that one possible laser source for this application would be an Er:YAG laser with output energy of over 50 mJ (and up to 500 mJ) and with pulse durations that are between 50 and 400 ns. By contrast, we have shown that considerably lower ablation fluence thresholds are possible with a pure photospallation mechanism which is exploited to best advantage when the pulse duration is shorter than 50 ns.
In our recent experiments, we determined that there is a significant dependence of ablation thresholds on the pulse duration and that thresholds lower than 100 mJ/cm.sup.2 are consistently feasible with pulses shorter than 50 ns. The significance of this finding is that it is possible to carry out a corneal ablation procedure using a mid-infrared short pulse laser with much lower energy outputs than previously taught. For example, with a pulse duration shorter than 50 ns, energy of less than 1 mJ per pulse in a 1 mm diameter spot size delivered to the cornea is sufficient to produce consistent ablation. Assuming losses in the beam delivery system of around 50%, less than 2 mJ of laser output per pulse is needed.
As suggested in our U.S. patent application Ser. No. 08/549,385, a Q-switched erbium-doped laser, operating directly at 2.94 .mu.m, becomes a practical option for such a laser source. A compact, reliable erbium laser has a number of desirable properties, including its simplicity of design, ease of maintenance and potentially low cost. While highly attractive, there were a number of factors which, to date, hindered the realization of an Er:YAG laser operating in the desired short pulse mode. In particular, it was believed that commercially available Q-switches based on Pockels cells with standard nonlinear materials such as LiNbO.sub.3 (lithium niobate) may not be appropriate for erbium-doped lasers due to unacceptably high absorption in both material and coatings near the 3 .mu.m wavelength, leading to low energy threshold for damage. On the other hand, alternative methods for Q-switching that rely on rotating prisms and mirrors, for example, those used by Lian & Wang in SPIE, 2393, pp. 160-166, 1995, or frustrated total internal reflection (FTIR), for example, as discussed by H. J. Eichler et al in Opt. Mat. 5, pp. 259-265, 1995 tended to result in pulse durations that were longer than 60 ns.
For the applications of interest, however, where ablation precision on the order of 1 micron are desired, a shorter pulse (shorter than 50 ns) is preferred, as such shorter pulses will increase the percentage of a true photospallative ablation process over a photothermal one, thus reducing residual contributions to tissue ablation from undesirable thermal effects to a minimum. It has been observed that an Er:YAG laser with the range of parameters taught by Lin will not result in the requisite sub-micron thermal effects and, hence, the shorter pulses recited above are important to achieving clinically successful ablation results in the mid-infrared region, similar to those of an excimer laser.
Due to limitations imposed by fundamental level dynamics and long upper-laser-level lifetimes, a practical lower limit on the pulse duration for a Q-switched erbium based laser is estimated to be about 20 ns. An electro-optic pulse switching means is, in principle, capable of achieving this pulse duration but has not yet been realized at useful energy output levels. For example, a lamp-pumped Er:YAG laser which was Q-switched electro-optically with a LiNbO.sub.3 -based modulator achieved only 7.5 mJ with pulse durations that were longer than 100 ns and at very low efficiency. For a more detailed discussion of such results, see, for example, E. Nava et al, SPIE vol. 2624, p. 246,1988. The main issue that prevented achievement of a higher energy and shorter pulse width in these experiments was damage to the optical components, including, but not limited to, the Q-switch material. While the FTIR Q-switch suggested by H. J. Eichler, as discussed above, may achieve a shorter pulse duration, on the order of 60 ns, it also limits the repetition rate to less than 10 Hertz, due to thermal loading and time constant characteristics.
The present invention discloses a novel approach to constructing an erbium-doped laser apparatus which overcomes the aforementioned difficulties and is capable of producing considerably shorter-pulse radiation with higher energy at or near 2.94 .mu.m. The apparatus is uniquely suited to performing PRK reliably and at low cost, thus greatly increasing the availability of the procedure to a larger number of people. Furthermore, with certain adjustments to the apparatus, it may be used for other ophthalmic procedures where a concentrated pulsed beam at a selected mid-IR wavelength has demonstrated benefits, such as in laser sclerostomy, trabeculectomy and vitreo-retinal surgery. Several such procedures were described in our companion U.S. pat. application Ser. No. 08/549,385, all of which are incorporated by reference herein. The recent emergence of fiber delivery systems for delivering mid-IR wavelengths may also provide further utility for the erbium short pulse laser in general endoscopic microsurgical applications, including in neural, orthoscopic and spinal cord surgery. Such medical procedures may derive great benefit from the highly localized effects generated by variants of the present system because of the delicate nature of the tissues involved.